The “Habsburg jaw”, a facial condition of the Habsburg dynasty of Spanish and Austrian kings and their wives, can be attributed to inbreeding, according to new results published in the Annals of Human Biology.
The new study combined diagnosis of facial deformities using historical portraits with genetic analysis of the degree of relatedness to determine whether there was a direct link.
The researchers also investigated the genetic basis of the relationship.
Generations of intermarriage secured the family’s influence across a European empire including Spain and Austria for more than 200 years but led to its demise when the final Habsburg monarch was unable to produce an heir.
However, until now no studies have confirmed whether the distinct chin known as “Habsburg jaw” was a result of inbreeding.
“The Habsburg dynasty was one of the most influential in Europe, but became renowned for inbreeding, which was its eventual downfall.
We show for the first time that there is a clear positive relationship between inbreeding and appearance of the Habsburg jaw,” says lead researcher Professor Roman Vilas from the University of Santiago de Compostela.
The researchers recruited 10 maxillofacial surgeons to diagnose facial deformity in 66 portraits of 15 members of the Habsburg dynasty.
Despite differences in artistic style, the portraits are characterised by a realistic approach to the human face.
The surgeons were asked to diagnose 11 features of mandibular prognathism, otherwise known as “Habsburg jaw”, as well as seven features of maxillary deficiency, the most recognisable of which are a prominent lower lip and an overhanging nasal tip.
The portraits, which can be viewed online, are preserved by some of the most important art museums in the world, including the Kunsthistorisches Museum in Vienna and the Prado Museum in Madrid.
The surgeons gave scores for the degree of mandibular prognathism and maxillary deficiency in each member of the Habsburg family. Mary of Burgundy, who married into the family in 1477, showed the least degree of both traits.
Mandibular prognathism was most pronounced in Philip IV, King of Spain and Portugal from 1621 to 1640.
Maxillary deficiency was diagnosed to the greatest degree in five members of the family: Maximilian I (regent from 1493), his daughter Margaret of Austria, his nephew Charles I of Spain, Charles’ great-grandson Philip IV and the last in the Habsburg line, Charles II.
The study authors detected a correlation between the two conditions, suggesting that “Habsburg jaw” is in fact characterised by them both and that they share a common genetic basis.
The extent of inbreeding was calculated from a large-scale family tree, including more than 6,000 individuals belonging to more than 20 generations.
Analysis was carried out to determine if it was connected to the degree of facial deformity. The researchers detected a strong relationship between the degree of inbreeding and the degree of mandibular prognathism.
The relationship to maxillary deficiency was also positive, but it was only statistically significant in two of the seven features diagnosed.
The causes of the relationship between inbreeding and facial deformity remain unclear, but the authors suggest it’s because the main effect of mating between relatives is an increase in the chances of offspring inheriting identical forms of a gene from both parents, known as genetic homozygosity. This reduces people’s genetic fitness, so “Habsburg jaw” should be considered a recessive condition.
However, the authors note that the study involves only a small number of individuals so it’s possible that the prevalence of Habsburg jaw is due to the chance appearance of traits, or genetic drift. They suggest this scenario is unlikely, but can’t rule it out.
“While our study is based on historical figures, inbreeding is still common in some geographical regions and among some religious and ethnic groups, so it’s important today to investigate the effects,” says Vilas. “The Habsburg dynasty serves as a kind of human laboratory for researchers to do so, because the range of inbreeding is so high.”
The causes of the extinction of the Neandertal populations in western Eurasia by 40,000 BP1 is a topic of intense debate in human evolution.
Some interpretations attribute this extinction to competition with early anatomical modern humans (AMHs), which would present differences expressed for instance through more efficient exploitation of dietary resources, possibly related to differential cognitive, behavioral and cultural abilities, that could rest on life-history and ontogenetic differences2–5.
However, other interpretations have recognized recent findings that support Neandertal dietary flexibility and multiple subsistence strategies6,7, increasing evidence of symbolic behavior and complex technologies8–13, and lack of fundamental differences in the overall pace of dental and skeletal growth and maturation in comparison with AMHs14, all of which complicate a scenario of AMHs simply outcompeting Neandertals15.
Environmental change also has been considered as a potential important factor in the Neandertal demise, whether acting independently, or in combination with other previously-mentioned differences between the two hominins16–18.
In the context of competition, the very fact of interbreeding within what has been called a hominin metapopulation19, would suggest a complex interaction between Neandertal, AMHs populations and Denisovans that has yet to be defined in detail20.
For instance, demographic and ecocultural modeling have included competition models, based on cultural and demographic differences21,22, and selectively-neutral models, based on migration dynamics and local dispersal and replacement alone (in absence of culturally-driven selection or environmental factors)23, with both resulting in the replacement of Neandertals by AMHs. Other models conclude that hunting-prey decline or climatic variations alone was not sufficient to cause the disappearance of Neandertals24.
In all cases, most researchers agree that, “whatever the extent to which the eventual replacement of late archaic human morphology involved admixture, absorption, and/or population displacement, the process was ultimately a demographic one”25.
In this regard, it has been suggested that the archaeological evidence supports substantial demographic differences at the Neandertal-to-AMH transition, with up to a tenfold increase in population density for early AMHs compared with Neandertals26, that could have been a critical factor in the Neandertal demise.
Although others have recommended caution when making inferences about population size from the archaeological record27, it is not unreasonable to suggest that demographic differences in population size and density, and in group size, could have been an important factor in the disappearance of Neandertals28.
In addition, the general demographic structure of Pleistocene Homo, with small effective population sizes (see below), a hunter-gatherer existence and population dispersal into separate small kindred groups, would have favored substantial levels of intragroup, and potentially intrafamily, mating29,30.
Important contributions to Neandertal paleodemography in this direction come from genetic studies, where high levels of inbreeding, or mating among relatives, and a general decrease in heterozygosity have been observed.
Specifically, Neandertals from the Altai, Vindija, Mezmaiskaya and El Sidrón sites present low levels of heterozygosity and small estimated effective population sizes averaging around 3000 individuals, both characteristics considered typical of archaic hominins, indicating that they lived in small and isolated populations31. Studies of genetic homozygosity indicate that Neandertals had a long history of high but variable levels of inbreeding.
The most extreme values are found in the Altai Neandertal, with long stretches of homozygosity that indicate recent inbreeding consistent with parental relatedness between two half-siblings31.
In contrast, Vindija Neandertal homozygosity is comparable to modern human groups like the Karitiana and Pima, suggesting that consanguinity was not ubiquitous among all Neandertal populations31.
At El Sidrón, a Neandertal sample (SD1253) had a larger cumulative length of homozygous genomic stretches of 10–100 Kb than samples from Vindija, Altai, Denisova, great apes and modern humans32, indicating a long history of inbreeding. In addition, the mitochondrial DNA (mtDNA) analysis of twelve El Sidrón individuals revealed low mtDNA genetic diversity and close kin relationships within the group33.
Within this context, it is reasonable to expect that a scenario of small, isolated groups of Pleistocene Homo with potentially high levels of intragroup mating would be also phenotypically expressed in the skeleton. For instance, recent analyses of bony labyrinth morphology in the Aroeira 3 cranium suggest a degree of demographic isolation in geographically and chronologically close hominins around the origin of the Neandertal clade34, and as previously suggested35 and recently shown36, there is a high incidence of developmental abnormalities and anomalies in Pleistocene Homo, several of them very rare or with unknown etiology. In past and present modern human populations, dental and skeletal anomalies and low-frequency anatomical variants have been associated with geographical isolation and/or endogamy37.
Given the nuclear and mtDNA genetic evidence that indicates that the 13 individuals from El Sidrón constitute a closely related kin group33, El Sidrón is the ideal Pleistocene sample to test for skeletal evidence of inbreeding.
Previous morphological analyses of the El Sidrón Neandertals have reported congenital clefts of the first cervical vertebra37 and the retention of a deciduous mandibular canine in two individuals38, but a systematic analysis of the entire sample has not yet been done. Here we present the results of the complete morphological analysis of the 1674 identified skeletal specimens from a total of 2556 remains recovered from El Sidrón.
More information:Annals of Human Biology, tandfonline.com/10.1080/03014460.2019.1687752
